A comparison of genetic connectivity in two deep sea corals to examine whether seamounts are isolated islands or stepping stones for dispersal

Abstract

Ecological processes in the deep sea are poorly understood due to the logistical constraints of sampling thousands of metres below the ocean's surface and remote from most land masses. Under such circumstances, genetic data provides unparalleled insight into biological and ecological relationships. We use microsatellite DNA to compare the population structure, reproductive mode and dispersal capacity in two deep sea corals from seamounts in the Southern Ocean. The solitary coral Desmophyllum dianthus has widespread dispersal consistent with its global distribution and resilience to disturbance. In contrast, for the matrix-forming colonial coral Solenosmilia variabilis asexual reproduction is important and the dispersal of sexually produced larvae is negligible, resulting in isolated populations. Interestingly, despite the recognised impacts of fishing on seamount communities, genetic diversity on fished and unfished seamounts was similar for both species, suggesting that evolutionary resilience remains despite reductions in biomass. Our results provide empirical evidence that a group of seamounts can function either as isolated islands or stepping stones for dispersal for different taxa. Furthermore different strategies will be required to protect the two sympatric corals and consequently the recently declared marine reserves in this region may function as a network for D. dianthus, but not for S. variabilis.

Figure 1

Map of four sampling regions (a) for the two deep sea coral species Desmophyllum dianthus and Solenosmilia variabilis in the Southern Ocean and details of individual seamounts (b) sampled within the Tasmanian Seamounts complex, noting five replicate sites were sampled on each of Mini Matt and Hill U seamounts. Maps were generated using Manifold GIS (V8.29) www.manifold.net based on the publically available Australia Bathymetry and Topography Grid, June 2009 obtained from Geoscience Australia (www.ga.gov.au).

Figure 2. Genetic groups within Desmophyllum dianthus, based on output from STRUCTURE analysis and grouped by collection depth.

Figure 3. Principal coordinates analysis showing three clear groups associated with collection depth within the Desmophyllum dianthus samples.

Figure 4. Genetic groups within Solenosmilia variabilis based on output from STRUCTURE analysis and grouped by seamount sampling location.

Figure 5. Schematic of gene flow among Tasmanian seamounts for Desmophyllum dianthus.

Arrows indicate where there is statistically significant directional gene flow between pairs of seamounts, with the arrow indicating the direction of the highest gene flow. The width of lines is directly proportional to the magnitude of gene flow, which ranges from N_em = 6.96 (Hill Z9 to Hill U) to N_em = 151.5 (Hill U to Dory Hill). Dashed lines show other instances where N_em >10 (i.e. sufficient to be considered ecologically connected). Red arrows indicate westerly flow, green arrows indicate easterly flow. The figure was created using Microsoft PowerPoint based on migration rates calculated in LAMARC.

Figure 6. Predicted dispersal of passive particles from the Tasmanian seamounts.

The dispersal probabilities were generated using “Connie2.0” CSIRO Connectivity Interface http://www.csiro.au/Connie2/ and are based on the sum across years 1993–2007. Dispersal was modelled at 995 m depth (the black cross marks release point), with a dispersal period of 25 days, and across the likely reproductive period for D. dianthus and S. variabilis in the Southern Ocean (January to May44). The figure was created in R version 3.1.2 using the packages raster, rasterVis and sp.